The reduction of dot gain in half-tone printing is desirable in order to accurately reproduce designs in printed format. An important contribution to dot gain is the spreading of ink on the substrate after image formation but prior to immobilisation by curing or drying. The spreading of a typical screen ink was modelled to determine the thermodynamic processes controlling the rate of drop spreading and the timescales over which they are significant. Suggestions are offered as to the characteristics desirable in polymer substrates in order to minimise dot spreading.

This study is original in that it derives from a modification of the approach taken by de Ruijters to the modelling of simple spherical drops. In this case a complex fluid is employed, namely a typical UV-curing ink, on four polymer surfaces. In this work the drops are considered to be driven towards their equilibrium geometry by a spreading force arising from surface tension effects. The spreading force is dissipated by the effects of viscous resistance to flow within the drop and molecular adhesion at the three-phase line. The model derived describes the rate of spreading in terms of three parameters, and once these parameters have been extracted results in being able to predict the extent of ink spreading and determine the rate of spreading. Although the study was carried out using screen inks it can be applied to most printing systems including ink–jet printing.

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